1. Field of the Invention
The invention relates to a reverse-input shutoff clutch that transmits positive and negative rotational torque from the input side to the output side and does not transmits positive and negative rotational torque from the output side to the input side.
2. Description of the Related Art
More and more electricity-assisted type trolleys are used as delivery trolleys for home delivery service because of the reasons such as, for example, increasing amount of goods loaded on trolleys, aging delivery personnel, and assimilating women in the industry. It is preferable in this case that trolleys can be electrically assisted for both forward and backward movements and also manually operated when the drive power source is stopped.
Such a function can be realized using, for example, the clutch disclosed in Japanese Patent Laid-Open Publication No. Hei. 8-177878, a proposal made by the present applicant.
The clutch disclosed in the publication is, as shown in FIG. 23, and FIGS. 24(a) and 24(b), provided with an outer ring 1, an inner ring 2, a torque transmission member 3 that can be engaged with and disengaged from the outer ring 1 and the inner ring 2 in both positive and negative rotation directions, a cage 4 for retaining, through its rotation relative to the outer ring 1, the torque transmission member 3 and for controlling the engagement and disengagement of the torque transmission means 3, a centering spring 5 for connecting the outer ring 1 and the cage 4 in a rotation direction, a stationary-side member 7 fixed to a stationary system in a rotation direction, and a viscous fluid 9 interposed between the cage 4 and the stationary-side member 7. The viscous fluid 9 is, for example, a viscous fluid, such as a silicone oil, for applying rotational resistance to the cage 4.
Rotational torque of a motor (not shown) is transmitted to the outer ring 1 through, for example, worm gear gearing 6. As shown in FIG. 24(a), a plurality of cam faces 1b are provided on the internal periphery of the inner ring 1 at equal spacing in a circumferential direction. Also, wedge gaps, symmetrical in both positive and negative rotation directions, are formed between the cam faces 1b and the external periphery of the inner ring 2. The torque transmission members 3 are, as shown in FIG. 24(a), cylindrically shaped rollers arranged in the wedge gaps, and received and retained in pockets 4a of the cage 4. As shown in FIG. 24(b), the centering spring 5 is coupled to the end faces of both the cage 4 and the outer ring 1, enabling the cage 4 to co-rotate with the outer ring 1. Further, the centering spring 5 serves to position the cage 4 so that the individual torque transmission members 3 are placed at the centers of the corresponding individual cam faces 1b of the outer ring 1. The rotation of the cage 4 with respect to the stationary-side member 7 causes the viscous fluid 9 to apply viscous shearing resistance to the cage 4, resulting in delay in rotation of the cage 4 with respect to the outer ring 1.
In this way, when positive or negative rotational torque is applied from the outer ring 1, as shown in FIG. 25(b), viscous shearing resistance of the viscous fluid 9 causes the cage 4 to delay in rotation with respect to the outer ring 1. As a result, the torque transmission member 3 is engaged with the corresponding wedge gap between the outer ring 1 and the inner ring 2. Consequently, rotational torque applied to the outer ring 1 is transmitted to the inner ring 2 through the torque transmission members 3. Contrary to this, when positive or negative rotational torque (reverse input torque) is applied from the inner ring 2 in reverse to the above, as shown in FIG. 25(a), the cage 4 is aligned to the outer ring 1 by the centering spring 5. This way the torque transmission member 3 is positioned at a center c1 in the circumferential direction of the cam faces 1b. In this case, the individual torque transmission members 3 are disengaged from their corresponding individual wedge gaps to be in a state where the members are uncoupled from both the inner ring 2 and the outer ring 1. Therefore, the reverse input torque applied to the inner ring 2 is not transmitted to the outer ring 1, and torque transmission is thus shut off.
As described above, the clutch shown in FIG. 23 to FIGS. 25(a) and 25(b) is constituted in such a way that viscous shearing resistance of the viscous fluid 9 interposed between the cage 4 and the stationary-side member 7 causes the cage 4 to delay in rotation with respect to the outer ring 1. In this constitution, the following problem is feared.
(1) As shown in FIG. 26, viscous shearing resistance (K2) of the viscous fluid 9 increases in proportion to the rotational speed (rotational angular speed) of the outer ring 1. Further, when the rotational speed of the outer ring 1 reaches a predetermined value and the viscous shearing resistance (K2) is at a torque (K1) at which the centering spring 5 is deformed by a specific amount, the torque transmission members 3 start to engage with the outer ring 1 and the inner ring 2. Then, rotational torque applied to the outer ring 1 is transmitted to the inner ring 2 through the torque transmission members 3.
Accordingly, in the clutch described above, the outer ring 1 is required to rotate at a speed equal to or higher than a specific rotational speed (rotational speed at which the engagement starts) to switch the state of the clutch to a torque transmission state. For this reason, the clutch requires a specific length of time after the rotation drive source starts to rotate until the inner ring starts to rotate, creating a problem of start-up response.
(2) The viscosity of the viscous fluid 9 varies depending on temperature, so that a rotational speed at which the engagement starts may change depending on ambient temperature of use.
(3) The viscous fluid 9 may deteriorate initially given functions of the clutch in long-term use even if the fluid is sealed with seals (such as lip seals or labyrinth seals). This is because friction force at the sealing portions may fluctuate to cause changes in rotational resistance of the seals against the cage or leakage of the viscous fluid.